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<Lakes and Reservoirs - Similarities, Differences and Importance> Water Quality The physical character and water quality of rivers draining into lakes and reservoirs are governed in part by the velocity and the volume of river water. The characteristics of the river water typically undergo significant changes as the water enters the lake or reservoir, primarily because its velocity reduces: sediment and other material carried in the faster-flowing water settle out in the basin, undergoing sedimentation. The structure of the biological communities also changes from organisms suited to living in flowing waters to those that thrive in standing or pooled waters. Greater opportunities for the growth of algae (phytoplankton) and the development of eutrophication are present too. Reservoirs typically receive larger inputs of water, as well as soil and other materials carried in rivers than lakes. As a result, reservoirs usually receive larger pollutant loads than lakes. However, because of greater water inflows flushing rates are more rapid than in lakes. Thus, although reservoirs may receive greater pollutant loads than lakes, they have the potential to flush the pollutants more rapidly than do lakes (as one can more rapidly flush water from a bathtub by increasing the water flow and/or the rate at which the water is drained from the tub). Reservoirs may therefore exhibit fewer or less severe negative water quality or biological impacts than lakes for the same pollutant load.
Many waste chemical compounds from industry, some with toxic or deleterious effects on humans and/or water-dependent products, are discharged into lakes and reservoirs. These can kill aquatic organisms and damage irrigated crops. Because of inadequate water purification the quantity of bacteria, viruses and other organisms in discharged waters are a primary cause of waterborne disease. Although dangerous to human health worldwide, such problems are particularly severe in developing countries.
Reservoirs, particularly the deeper ones, are also distinguished from lakes by the presence of a longitudinal gradient in physical, chemical and biological water quality characteristics from the upstream river end to the downstream dam end. Thus, reservoirs have been characterized as comprizing three major zones: an upstream riverine zone, a downstream lake-like zone at the dam end, and a transitional zone separating these two zones (Fig. 4). The relative size and volume of the three zones can vary greatly in a given reservoir.
Downstream Characteristics
Constructing a reservoir to protect downstream areas from floods often has significant social and economic implications, including the potential for stimulating urban and agricultural development adjacent to, and below, the reservoir. This can have both positive and negative impacts, depending on the nature and size of development.
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The water quality of lakes
and reservoirs is defined by variables measured within the water basin (Photo
12). Although there are many variables of limnological significance, water quality
is typically characterized on the basis of such variables as water clarity or
transparency (greater water clarity indicates better water quality), concentration
of nutrients (lower concentrations indicate better water quality), quantity of
algae (lower levels indicate better water quality), oxygen concentration (higher
concentrations are preferred for fisheries), concentration of dissolved minerals
(lower values indicate better water quality), and acidity (a neutral pH of 7 is
preferred).
Major differences
between deep and shallow water-bodies, whether lakes or reservoirs, occur. Deep
lakes, particularly in non-tropical regions, usually have better water quality
in lower layers. Shallow lakes (Photo 13) do not exhibit this depth differentiation
in quality and their more shallow, shoreline areas have relatively poorer water
quality because they are where pollutant inputs are discharged and areas with
a greater potential for disturbance of bottom muds, etc. Thus, the water quality
of a natural lake usually improves as one moves from the shoreline to the deeper
central part. In contrast, the deepest end of a reservoir is immediately upstream
of the dam so that water quality usually improves along the length of a reservoir,
from the shallow inflow end to the deeper, “lake-like” end near the dam.
Constructing
a dam can produce dramatic changes in the downstream river channel below the dam
that are quite unlike downstream changes from lakes. Because reservoirs act as
sediment and nutrient traps, the water at the dam end of a reservoir is typically
of higher quality than water entering the reservoir. This higher-quality water
subsequently flows into the downstream river channel below the dam. This phenomenon
is sometimes a problem in that the smaller the quantity of sediments and other
materials transported in the discharged water, the greater the quantity that can
be picked up and transported as it moves downstream. Because it contains less
sediment the discharged water can scour and erode the streambed and banks as it
picks up new sediment as it continues downstream. This scouring effect can have
significant negative impacts on the flora, fauna and biological community structure
in the downstream river channel. The removal of sediments from a river by reservoirs
also has important biological effects, particularly on floodplains.
Many reservoirs,
especially those used for drinking supplies, have water release or discharge structures
located at different vertical levels in their dams (Fig. 5). This allows for the
withdrawal or discharge of water from different layers within the reservoir, so
called “selective withdrawal”
(Photo 14). Depending on the quality of the water
discharged, selective withdrawal can significantly affect water quality within
the reservoir itself, as well as the chemical composition and temperature of the
downstream river. The ability to regulate or schedule water and silt discharges
(Photos 15, 16 and 26) also can fundamentally change downstream hydrological regimes,
affecting both flora and fauna.
